[SwiftmailerBridge] Bump allowed versions of swiftmailer

Current version of Swiftmailer is 5.2.1, while (previously to this commit)
the version installed by composer was 5.0.3.

This is rather important, since 5.2.1 closes a security issue that 5.0.3 is
vulnarable to (swiftmailer/swiftmailer#494).

src/Symfony/Bridge/Swiftmailer/composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.3",
-        "swiftmailer/swiftmailer": ">=4.2.0,<5.1-dev"
+        "swiftmailer/swiftmailer": ">=4.2.0,<6.0-dev"
     },
     "suggest": {
         "symfony/http-kernel": ""